1. Field of the Invention
The present invention relates to a combustion method and apparatus for reducing emission concentrations of NO.sub.x (nitrogen oxides) and CO (carbon monoxide), which is suitable for use in water-tube boilers such as once-through boilers, natural circulation water-tube boilers, and forced circulation water-tube boilers.
2. Description of the Prior Art
In recent years, there has been a demand for further reducing emission concentrations of harmful combustion exhausts, particularly of NO.sub.x and CO, also in boilers from the environmental pollution's viewpoint and the like. There have already been proposed various types of measures for reducing the emission concentrations of such harmful combustion exhausts. As one of the measures for the reduction, there is known from U.S. Pat. No. 5,020,479 a technique that heat absorbing tubes are brought as close as possible to the burner combustion surface so that the group of heat absorbing tubes are positioned in the combustion flame, wherein heat exchange and flame cooling are simultaneously effected to thereby suppress generation of thermal NO.sub.x and moreover realize high-load combustion. It is noted that "combustion flame" herein used refers to high-temperature gas that is under progress of combustion reaction, the high-temperature gas including combustible premixed gas, which has not yet burned completely, and burnt gas, which has been generated as a result of combustion. Also, the term combustion flame can be replaced by combustion gas.
However, this conventional measure, although capable of reducing the emission concentration of NO.sub.x, results in a slightly high emission concentration of CO which is a problem. One cause of this, it is suspected, is that the cooling of combustion flame to be rendered for NO.sub.x reduction in turn produces a rapid cooling effect upon CO, thereby freezing the reaction such that part of the combustion gas is discharged outside the system as unreacted substances, i.e. CO and others, remaining at its equilibrium concentration. To solve this problem, proposed in Japanese Patent Laid-Open Publication SHO 60-78247 was a technique in which after flame temperature is controlled to above 1000.degree. C. and below 1500.degree. C. by a cold substance placed in proximity to or in contact with a flame generated by high-load combustion, residual CO in the flame is oxidized in an adiabatic space provided downstream of the cold object, thus being transformed into CO.sub.2 (carbon dioxide).
However, this technique is intended to reduce the emission of CO, and not to suppress the generation of NO.sub.x. For this reason, the adiabatic space temperature of NO.sub.x may increase depending on where the adiabatic space is located, such that NO.sub.x will be generated. Also, there is another problem that temperature rise of the boiler body wall that defines the adiabatic space may become large, depending on the conditions under which the adiabatic space is formed. To prevent this temperature rise, it is necessary to provide a thermal insulant on the inner surface of the boiler body wall of the adiabatic space side, which leads to an increase in system cost. Further, when the thermal insulant is provided, there may arise a possibility that the thermal insulant may drop over long-term use. Furthermore, with a high flow velocity of the combustion flame, necessary transformation of CO to CO.sub.2 can be accomplished by ensuring an elongated length of the adiabatic space in the direction of combustion flame flow, whereas in this case the thermal efficiency is reduced, such that the boiler body cannot be reduced in size, unfavorably.